1. Field of the Invention
The present invention relates to a high-frequency power amplifier and in particular to a high-frequency power amplifier used in a wireless LAN terminal and a mobile portable terminal.
2. Description of the Related Art
As a device excellent in high-frequency characteristics, a hetero-junction bipolar transistor (hereinafter referred to as the HBT) is used in a high-frequency power amplifier. Recently, to miniaturize wireless LAN terminals or mobile portable terminals, high-frequency power amplifier comprising HBTs is integrated, for example, into a MMIC (Microwave Monolithic IC) comprising high-frequency amplification transistors and bias circuits. An example that aims to enhance the characteristics of a high-frequency power amplifier including a bias circuit is a high-frequency power amplifier described in Patent Reference 1. FIG. 33 shows the structure of the high-frequency power amplifier.
In the circuit shown in FIG. 33, a bias supply transistor 41 is connected to the base of a high-frequency power amplifying transistor 31. To the base of the bias supply transistor 41 is connected a bias circuit for supplying a bias voltage composed of resistors 51, 52. Further, between the connection point of the bias supply transistor 41 and the bias circuit and the reference potential is connected a capacitor 61. Between an input terminal 01 of a high-frequency power amplifier and the high-frequency power amplifying transistor 31 as well as between an output terminal 02 and the high-frequency power amplifying transistor 31 are respectively inserted matching circuits 11, 12.
In this circuit configuration, the power amplitude of the base of the high-frequency power amplifying transistor 31 is greater in high-output operation, so that the bias supply transistor 41 is unstable at high frequencies. That is, the high-frequency signal of a high-frequency amplifying transistor influences the base potential of the bias supply transistor 41. As a result, the voltage across the base and emitter of the bias supply transistor 41 drops and the current supply to the high-frequency power amplifying transistor 31 is likely to decrease.
Accordingly, as the output amplitude of the high-frequency power amplifying transistor 31 tends to increase, the operating point drops at the same time and high output is likely to result in gain compression. With the circuit shown in FIG. 33, the capacitor 61 instantaneously supplies a current to the base of the bias supply transistor 41 by way of charging or discharging a current to reduce a high-frequency-based drop in the voltage applied to the base of the high-frequency power amplifying transistor 31, thereby maintaining the operating point and having an efficient effect on the gain compression.
However, with the high-frequency power amplifying transistor 31 or bias supply transistor 41, a temperature rise due to heat in operation causes variations in the base-transmitter voltage for optimum operation. The bias circuit designed based on simple resistive potential division as shown in FIG. 33 cannot compensate for such variations and characteristics of the transistor substantially vary with variations in temperature.
An example of a bias circuit featuring temperature compensation is a high-frequency power amplifier described in Patent Reference 2, whose structure is shown in FIG. 34. Referring to FIG. 34, the high-frequency power amplifier comprises a high-frequency power amplifying transistor 31, a first temperature compensating transistor 42 for supplying to the high-frequency power amplifying transistor 31 a current corresponding to a voltage applied to a bias voltage supply terminal, and a second temperature compensating transistor 43 for correcting a bias current supplied from the bias supply transistor 41 to the high-frequency power amplifying transistor 31 in accordance with a current flowing through the first temperature compensating transistor. Resistors 51, 52, 54 are for regulating a bias current. A numeral 01 represents the input terminal of a high-frequency power amplifier, 02 the output terminal of an high-frequency power amplifier, 21 a power source terminal, and 22 a control input terminal to which a control signal inputted from outside.
In this circuit configuration, a bias circuit is connected that is composed of the temperature compensating transistors 42 and 43 as well as the resistors 52, 54 for bias regulation. This suppresses variations in the bias current of am amplifier caused by variations in the input voltage.
Patent Reference 1: Japanese Patent No. 3,377,675, Specification (FIG. 4 on Page 6)
Patent Reference 2: JP-A-2002-9558
The voltage supplied to a power source is typically a voltage stabilized by a regulator, so that the variation width of a voltage is relatively small. For example, in case a voltage of 2.8V is supplied and the voltage variation of the regulator is 5 percent, the resulting voltage width is approximately 0.3 volts from 2.66V to 2.94V.
However, in case a stabilized voltage is not given by the regulator for some reason or in case a regulated voltage is not supplied into the device, power is directly supplied from a battery as a power source. This results in a greater variation width of voltage. In the related art circuit shown in FIG. 33, stability against the variation of a power voltage is not provided so that there arises a conspicuous problem of degradation of linearity attributable to a drop in a current in the high-output operation.
In the related art circuit show in FIG. 34, leakage of a high-frequency signal from the high-frequency power amplifying transistor 31 to the bias supply transistor 41 in the high-output operation causes the base potential of the bias supply transistor to vary at high frequencies, which causes a problem of degradation of linearity.
To be more precise, in case a high-frequency power amplifying transistor includes multiple stages, a bias supply transistor similar to the bias supply transistor 41 is installed in multiple stages based on the bias voltage determined by the temperature compensating transistor 42 and 43, as shown in FIG. 35.
In FIG. 35, bias supply transistors 41, 44, 47 are installed in correspondence to the high-frequency power amplifying transistors 31, 32, 33 respectively. All the bias supply transistors use the bias voltage of a bias circuit as a reference.
With this configuration, in case the base potential of the temperature compensating transistor 42 that determines the reference voltage of all circuit operations, that is, the bias voltage varies at high frequencies, all the bias supply transistors and all the high-frequency power amplifying transistors are influenced.